Project summary/abstract: Osteoarthritis (OA) is associated with past biomechanical injury to cartilage and with aging. However, biomechanical injury and aging do not inevitably lead to OA. This proposal will focus on chondrocyte-centered innate inflammatory mechanisms in OA. We have characterized cartilage catabolism- promoting effects on chondrocytes for inflammation-mediated release of the multifunctional protein transglutaminase 2 (TG2). TG2 is abundant in the OA cartilage extracellular matrix and TG2 transamidation catalytic activity increases in aging cartilage. We observed that TG2 is a biomarker of OA severity, and we validated that global TG2 knockout is protective for instability-induced knee OA in mice. To pinpoint biomechanical injury and stress induced cartilage innate inflammatory responses that provide a foundation upon which OA is then triggered or accelerated, we will examine the linkage of TG2 release and XBP1 (X-box binding protein 1) activation in chondrocyte innate inflammation, proposed by us as a central switching mechanism for OA development and progression. XBP1 activation is specific to activation of ER stress, and successful resolution of ER stress promotes cell recovery and survival from injury. However, XBP1 activation also plays a central role in multiple organ models of innate inflammation, including via Toll-like receptor activation. We observe that biomechanical injury induces activation of ER stress in chondrocytes. Moreover, we detect active XBP1 increased in human knee OA chondrocytes. We also observe that cultured TG2 deficient chondrocytes have decreased XBP1 activation, but that extracellular TG2 induces XBP1 activation. Furthermore, we have generated cartilage-specific XBP1 knockout mice to probe for XBP1 function in chondrocytes. Here, we will test a hypothetical model in which TG2 and XBP1, by transducing and/or amplifying responses to biomechanical injury and oxidative stress, switch on chondrocyte innate inflammatory stress to promote OA development and progression. We specifically aim to: 1. Test the hypothesis that TG2 increases biomechanical injury-induced and oxidative stress-related catabolic and apoptotic responses of cultured chondrocytes mediated by HIF-2alpha activation and TG2 guanine nucleotide binding. 2. Test the hypothesis that TG2 increases injury and oxidative stress-induced catabolic and apoptotic responses of cultured chondrocytes critically mediated by XBP1 activation and decreased AMPK activity. 3. Test the hypothesis, in complementary studies in mice, that both cartilage-specific TG2 and XBP1 deficiency are protective against development and progression of OA. Completion of these studies will shed new light on the pathogenesis of OA related to biomechanical injury and aging, and identify potential new therapeutic targets.